Quantification of conservative endurance times in thermally insulated cold-stressed digits

Analytical expressions for estimating endurance time and glove thermal resistance related to human finger in cold conditions

Frostbite is considered the severest form of cold injury and can lead to necrosis and loss of peripheral appendages. Therefore, prediction of endurance time of limb's tissue in cold condition is not only necessary but also crucial to estimate cold injury intensity and to choose appropriate clothing. According to the previous work which applied a 3-D thermal model for human finger to analyze cold stress, in this study, an expression is presented for endurance time in cold conditions to prevent cold injury. A formula is also recommended to select a proper glove with specific thermal resistance based on the ambient situation and cold exposure time. By employing linear extrapolation and real physical conditions, the proposed formulas were drawn out from numerical simulation. Analytical results show good agreement with numerical data. The used numerical data had been also validated with experimental data existed in the literature. Furthermore, the effect of different parameters such as glove thermal resistance and ambient temperature is investigated analytically.

A 3D thermal model to analyze the temperature changes of digits during cold stress and predict the danger of frostbite in human fingers

The existing computational models of frostbite injury are limited to one and two dimensional schemes. In this study, a coupled thermo-fluid model is applied to simulate a finger exposed to cold weather. The spatial variability of finger-tip temperature is compared to experimental ones to validate the model. A semi-realistic 3D model for tissue and blood vessels is used to analyze the transient heat transfer through the finger. The effect of heat conduction, metabolic heat generation, heat transport by blood perfusion, heat exchange between tissues and large vessels are considered in energy balance equations. The current model was then tested in different temperatures and air speeds to predict the danger of frostbite in humans for different gloves. Two prevalent gloves which are commonly used in cold climate are considered for investigation. The endurance time and the fraction of necrotic tissues are two main factors suggested for obtaining the response of digit tissues to different environmental conditions.

Thermoregulatory modeling for cold stress

Modeling for cold stress has generated a rich history of innovation, has exerted a catalytic influence on cold physiology research, and continues to impact human activity in cold environments. This overview begins with a brief summation of cold thermoregulatory model development followed by key principles that will continue to guide current and future model development. Different representations of the human body are discussed relative to the level of detail and prediction accuracy required. In addition to predictions of shivering and vasomotor responses to cold exposure, algorithms are presented for thermoregulatory mechanisms. Various avenues of heat exchange between the human body and a cold environment are reviewed. Applications of cold thermoregulatory modeling range from investigative interpretation of physiological observations to forecasting skin freezing times and hypothermia survival times. While these advances have been remarkable, the future of cold stress modeling is still faced with significant challenges that are summarized at the end of this overview.

Raynaud's phenomenon: Infrared functional imaging applied to diagnosis and drug effects

A non-invasive, innovative approach to the study of Raynaud's Phenomenon is proposed. A group of patients, with respect of a control group, underwent a simultaneous assessment of thermal properties of all ten fingers using infrared functional imaging (IRFI). The assessment highlighted a quite different behaviour between patients with Primary- (PRP) and those with scleroderma - Raynaud's Phenomenon (SSc) and, compared with other existing techniques, seems to be an objective and effective tool to discriminate between PRP and RP secondary to SSc. 18 healthy volunteers (Norm), 20 Primary Raynaud's Phenomenon (PRP) and 20 Secondary Scleroderma (SSc) patients were studied subsequently to clinical evaluation and nail fold capillaroscopy. High-resolution infrared imaging of finger re-warming processes, immediately after a 2 min cold stress, allowed to identify objective parameters. Temperature integral Q (the temperature evaluation of the area under the time-temperature curve along the re-warming period) provided particularly effective figures in describing thermal properties of the fingers. Grand average Q values were (383.4 ∓ 12.5) °C×min, (502.9 ± 88.1) °C×min and (1022.0 ± 110.2) °C×min for the PRP, SSc and Normal groups, respectively. Separate evaluation of the temperature integral for each finger leads to very similar results for the fingers of all the PRP patients; a different thermoregulatory response was observed in SSc patients. The sensitivity of the method in order to distinguish healthy from ill fingers was 100%. The specificity in distinguishing SSc from PRP was 95%. In addition, IRFI parameters provided a better understanding of the impaired control of the finger's temperature in PRP and SSc with respect to the Normal group. This pilot study also applied IRFI for the measurement of drug effects in patients with Raynaud's Phenomenon. Sixteen out of twenty SSc patients were tested in a single 1-hour session of N-acetylcysteine infusion. IRFI clearly documented a significant increase of face and hands temperature during the drug administration. The grand average value of the finger's temperature after the 1 hour NAC administration was (29.6 ± 3.7) °C, while its value before was (27.9 ± 3.7) °C (p<0.001). N-acetylcysteine seems to act as a vasodilator in patients with Raynaud's phenomenon secondary to systemic sclerosis (scleroderma).

Evaluation of hand and finger heat loss with a heated hand model

A heated full-scale hand model has been used to determine indirectly hand and finger heat losses of human subjects exposed to four ambient cold conditions (0, 4, 10 and 16 degrees C, air velocity approximately 0.3 m/s). Heat transfer coefficients determined with the hand model, were used to calculate heat flux based on measured skin to ambient temperature gradients. The responses of eight subjects from a previous study were used for the analysis. The measurements were carried out in a small climate chamber which was cooled by evaporating liquid carbon dioxide. The thermal hand was put into the chamber in a vertical position with the thumb up. The surface temperature of the thermal hand was controlled at 21, 25, 28, 31 and 34 degrees C under each of the four ambient cold conditions, in order to investigate possible temperature dependence of the calculated combined convective and radiate heat transfer coefficient (hCR). The value of hCR varied between approximately 9-13 W/m2 degree C for fingers and palm and back of hand, respectively. Calculated heat losses showed significant individual variation, corresponding to the maintained skin to ambient temperature gradient. Individual values from about 50 to more than 300 W/m2 were calculated. Several subjects showed CIVD and heat fluxes associated with this phenomenon were sometimes doubled. The measurement results showed realistic and comparable with literature date. The advantages of the thermal hand model can be counted as easy to use; directly measures the heat loss; highly reproducible and no interruption. It appears that a heated hand model provides a useful methods for analysis and quantification of hand heat loss.

On the thermal efficiency of cold-stressed fingers

Thermal efficiency of cold-stressed finger-tips during cold induced vasodilatation (CIVD) is considered. The actual heat loss from the finger-tip is compared to either the minimal or the maximal heat losses. The actual heat loss is estimated by integrating the area under the time-temperature curve of the finger-tip. The minimal heat loss is estimated by extrapolating an exponential approximation of finger-tip temperature until it reaches a certain minimal value. The value used in this study is 5 degrees C, which is the pain threshold. The maximal heat loss is calculated by assuming finger-tip temperature to be maintained at its initial value throughout the cold exposure. These quantities were calculated for a series of exposures involving two environmental conditions of gloved subjects: Tdry bulb = -17.2 degrees C, Tdew point = -25.1 degrees C (cold-dry) and Tdry bulb = 0 degree C, Tdew point = -8.4 degrees C (cold-wet). Thermal efficiency was in the range of 0.40-0.85 for the minimal heat loss value (eta min) and 0.22-0.72 for the maximal heat loss value (eta max). Weak linear relationships between the two definitions of the thermal efficiencies and the total duration of the CIVD phase was indicated. The thermal efficiency based on minimal heat loss indicated an inverse relation with the total duration of the CIVD phase. This contradiction could be reconciled by the application of the common concept of "coefficient of performance". Considerable inter- and intra-subjects variability was found.

Simulation of a cold-stressed finger including the effects of wind, gloves, and cold-induced vasodilatation

The thermal response of fingers exposed to cold weather conditions has been simulated. Energy balance equations were formulated, in a former study, for the tissue layers and the arterial, venous, and capillary blood vessels. The equations were solved by a finite difference scheme using the Thomas algorithm and the method of alternating directions. At this stage of development the model does not include any autonomic control functions. Model simulations assumed an electrical heating element to be embedded in the glove layers applied on the finger. A 1.3 W power input was calculated for maintaining finger temperatures at their pre-cold exposure level in a 0 degree C environment. Alternate assumptions of nutritional (low) and basal (high) blood flows in the finger demonstrated the dominance of this factor in maintaining finger temperatures at comfortable levels. Simulated exposures to still and windy air, at 4.17 m/s (15 km/h), indicated the profound chilling effects of wind on fingers in cold environments. Finally, the effects of variable blood flow in the finger, known as "cold-induced vasodilatation," were also investigated. Blood flow variations were assumed to be represented by periodic, symmetric triangular waves allowing for gradual opening-closing cycles of blood supply to the tip of the finger. Results of this part of the simulation were compared with measured records of bare finger temperatures. Good conformity was obtained for a plausible pattern of change in blood flow, which was assumed to be provided in its entirety to the tip of the finger alone.

Numerical analysis of an extremity in a cold environment including countercurrent arterio-venous heat exchange

A model of the thermal behavior of an extremity, e.g., a finger, is presented. The model includes the effects of heat conduction, metabolic heat generation, heat transport by blood perfusion, heat exchange between the tissue and the large blood vessels, and arterio-venous heat exchange. Heat exchange with the environment through a layer of thermal insulation, depicting thermal handwear, is also considered. The tissue is subdivided into four concentric layers simulating, from the center outward, core, muscle, fat, and skin. Differential heat balance equations are formulated for the tissue and for the major artery and the major vein traversing the finger. These coupled equations are solved numerically by a finite-difference, alternating direction method employing a Thomas algorithm. The numerical scheme was extensively tested for its stability and convergence. This paper presents the model equations and results of the convergence tests, and shows plots of blood and tissue temperatures along the axis of the model for combinations of parameters including the effect of countercurrent heat exchange between the artery and the vein.

Windchill and the risk of tissue freezing

Low air temperatures and high wind speeds are associated with an increased risk of freezing of the exposed skin. P. A. Siple and C. F. Passel (Proc. Am. Phil. Soc. 89: 177-199, 1945) derived their windchill index from cooling experiments on a water-filled cylinder to quantify the risk of frostbite. Their results are reexamined here. It is found that their windchill index does not correctly describe the convective heat transfer coefficient (hc) for such a cylinder, the effect of the airspeed (v) is underestimated. New risk curves have been developed, based on the convection equations valid for cylinders in a cross flow, hc infinity v0.62, and tissue freezing data from the literature. An analysis of the data reveals a linear relationship between the frequency of finger frostbite and the surface temperature. This relation closely follows a normal distribution of finger-freezing temperatures, with an SD of 1 degree C. As the skin surface temperature falls from -4.8 to -7.8 degrees C, the risk of frostbite increases from 5 to 95%. These data indicate that the risk of finger frostbite is minor above an air temperature of -10 degrees C, irrespective of v, but below -25 degrees C there is a pronounced risk, even at low v.

Work in the cold. Review of methods for assessment of cold exposure

The obvious hazard of a cold exposure under natural as well as artificial conditions is tissue cooling and the associated sequel of more or less harmful effects from cold injury to discomfort. The nature, risk and magnitude of effects depend largely on the cooling effect, which results from the interaction of climatic factors (air temperature, mean radiant temperature, humidity and wind), protection (clothing) and metabolic heat production (activity). Assessment of cold stress should be based on methods which measure or predict this cooling effect in a relevant and reliable way. The nature of cooling encompasses (1) whole-body cooling, (2) extremity cooling, (3) convective cooling (wind chill), (4) conductive cooling (contact) and (5) airway cooling. The review contains a description of methods for evaluation of the various types of cold stress, as well as a discussion of their capacity and limitations. On the basis of selected methods, recommendations related to lowest permissible temperatures and other measures are discussed and compared with published data. Apparently, local cooling in most cases produces discomfort and harmful effects, before more significant whole-body cooling develops. With strong wind or movement at very low temperature, frostnip of unprotected skin may quickly develop. For most other conditions extremity (digit) cooling determines duration of exposure. However, as digit cooling largely depends on whole-body heat balance, it is important to control body cooling by selection and use of appropriate protective clothing.